1. Field of the Invention
The present invention relates to a write-once optical information recording medium capable of recording and reproducing information signals with high signal quality by irradiating a thin film formed on a substrate with a high energy light beam such as a laser beam, a method for producing the same, and a method and an apparatus for recording information thereon.
2. Description of the Related Art
A recording medium is known that allows information to be recorded and reproduced by irradiating a thin film formed on a transparent substrate with a laser beam focused on a tiny spot. As a write-once recording medium, a recording medium is known that includes a substrate and a recording thin film of TeOx (0 less than x less than 2), which is a mixture of Te and TeO2, formed on the substrate (JP 50(1975)-46317 A). This recording medium can provide a larger change in reflectance by irradiation of a light beam for reproduction.
The TeOx recording thin film is produced in the amorphous state and can form a crystalline recording mark by laser beam irradiation, thus eliminating an initialization process such as laser annealing. This is an irreversible process that makes it impossible to modify or erase information by overwriting. Therefore, a recording medium including the above recording thin film can be utilized as a write-once recording medium. The recording thin film does not require a dielectric protective layer or the like because of its high durability, such as high resistance to humidity. To reduce the manufacturing cost, the TeOx recording thin film is used as a single-layer film.
For the TeOx recording thin film, it takes some time until a signal is saturated after recording, i.e., crystallization in the recording thin film caused by laser beam irradiation proceeds sufficiently. Therefore, basically, the recording medium including such a recording thin film is unsuitable as a recording medium that needs to provide high-speed response, e.g., a data file for computers, in which data are recorded on a disk and then examined after the disk makes one revolution. To overcome this disadvantage, the addition of Pd and Au to TeOx as third elements has been proposed (JP 60(1985)-203490 A, JP 61(1986)-68296 A, and JP 62(1982)-88152 A).
Pd and Au are considered to accelerate the growth of crystals in a TeOx thin film when the film is irradiated with a laser beam. This allows crystal grains of Te and Te Pd alloy or Te Au alloy to grow rapidly. Pd and Au have high oxidation resistance, and thus they do not degrade the superior humidity resistance of the TeOx thin film.
As a basic means for increasing the amount of information stored per recording medium, the following is known: the spot size of a laser beam is reduced by using the laser beam having a shorter wavelength and an objective lens having a greater numerical aperture for focusing the laser beam. Also, mark edge recording or land and groove recording has been introduced. Moreover, a multi-layer structure in which a plurality of information layers are stacked has been proposed, and a layer recognition means and a layer switching means for selecting one of those information layers of the multi-layer structure also have been proposed.
To achieve high-density recording described above, the composition of a recording material containing TeOx and the additives of Pd and Au as the third elements, and a recording medium with an improved thickness have been proposed as well (JP 9(1998)-326135 A, WO 98/09823).
In general, it is necessary that a recording medium should have at least a certain predetermined reflectance before recording to perform servo control such as focusing or tracking. For a reflectance-increase-type medium, whose reflectance after recording is higher than that before recording, the reflectance is increased further after recording. Therefore, in this medium, either one of the reflectances before and after recording cannot be close to zero, which leads to a disadvantage in providing larger signal contrast. On the other hand, a reflectance-decrease-type medium can reduce the reflectance after recording while keeping the reflectance before recording high. Therefore, it is preferable in terms of servo control and also has the advantage of providing larger signal contrast.
However, every conventional recording medium including a TeOx recording thin film is a reflectance-increase type, whose reflectance after recording is higher than that before recording.
So far no consideration has been given to produce a reflectance-decrease-type recording medium including a TeOx recording thin film. This is probably caused by the fact that TeOx is a reflectance-increase-type material, i.e., a material whose reflectance increases with crystallization, even if the composition or thickness is changed. However, the present invention provides a reflectance-decrease-type recording medium including the TeOx recording thin film.
A recording medium of the present invention includes a transparent substrate and at least one information layer formed on the transparent substrate. The information layer includes a recording layer made of a material that includes Te, O and M, where M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au and Bi. A reflectance with respect to a light beam incident from the side of the transparent substrate after recording information on the recording layer is lower than a reflectance before recording.
A reflectance-decrease-type recording medium including the above recording thin film can be provided, e.g., by adding at least one reflectance adjustment layer to the information layer. Examples of the reflectance adjustment layer include a reflective layer and a dielectric layer, which will be described later. In the absence of the reflectance adjustment layer, a reflectance after recording information on the recording layer is typically higher than that before recording. However, the presence of the reflectance adjustment layer allows reflections from the medium to be adjusted due to the optical interference effect, so that a reflectance after recording is lower than that before recording. The reflectance adjustment layer can be a single layer or composed of a plurality of sub-layers.
The present invention further provides a method for producing the above recording medium, a recording method, and a recording apparatus. In an embodiment of a method for producing the recording medium of the present invention, an information layer is formed on a protective substrate, and then a transparent substrate is formed on the information layer. The method for providing the information layer on the protective layer as described above is suitable, e.g., for a thin transparent substrate having a thickness of not more than 0.4 mm.
In another embodiment of a method for producing the recording medium of the present invention, an annealing process is performed at a temperature of not less than 60xc2x0 C. for not less than five minutes after forming at least the recording layer. This method can provide a higher C/N ratio.
The recording method of the present invention includes recording information on a recording medium that moves relative to a light beam by irradiating the recording medium with the light beam from the side of the transparent substrate while modulating the light beam between a peak power and a smaller power than the peak power so as to form a recording mark. The product VT of a linear velocity V of the recording medium with respect to the light beam and a time T to maintain the light beam for irradiation at the peak power is determined, and the time T is set so that the product VT increases with an increase in the linear velocity V in terms of unit length of the recording mark.
The recording apparatus of the present invention includes the following: a rotation means for rotating the recording medium; an irradiation means for irradiating the recording medium that is rotated by the rotation means with a light beam to form a recording mark; and a modulation means for modulating the light beam emitted from the irradiation means between a peak power and a smaller power than the peak power. The product VT of a linear velocity V of the recording medium with respect to the light beam and a time T to maintain the light beam for irradiation at the peak power is determined, and the modulation means sets the time T so that the product VT increases with an increase in the linear velocity V in terms of unit length of the recording mark.
The thermal interference between the recording marks decreases with an increase in the linear velocity of the medium. Consequently, when the recording marks are formed to have the same length in the same period of time of irradiation of peak power, the recording marks become smaller as the linear velocity increases. According to the recording method and apparatus of the present invention, the irradiation time T is set so that the product VT of the linear velocity V of the recording medium and the irradiation time T of peak power is increased with an increase in the linear velocity V in terms of unit length of the recording mark to be recorded. This makes it possible to compensate the difference in size of the recording marks.